This invention relates to silylmethanethiols and their use as promoters in the reaction of hydroxyaromatic compounds with aldehydes and ketones in the presence of an acidic catalyst to afford bisphenols, such as bisphenol A (BPA).
Bisphenols, as exemplified by BPA, are widely employed in the manufacture of polymeric materials and are typically prepared by condensation of a hydroxyaromatic compound with an aldehyde or ketone in the presence of an acidic catalyst. Bisphenol A (BPA) is the principal monomer used in the manufacture of bisphenol A polycarbonate, a commercial engineering thermoplastic material. The manufacture of bisphenol A (BPA) from acetone and phenol is practiced globally on a large scale with hundreds of millions of pounds of BPA produced annually. Typically, phenol is reacted with acetone in the presence of an acidic catalyst and a thiol promoter. The thiol promoter acts to improve the rate and selectivity of BPA formation in the acid catalyzed condensation of phenol with acetone. Many different combinations of acidic catalysts and thiol promoters have been investigated and some thiol promoters such as 3-mercaptopropionic acid have been employed in the commercial scale production of BPA. Notwithstanding earlier research efforts and their attendant impressive process improvements in the manufacture of bisphenols such as bisphenol A, there is a continuing need to improve further both the rate and selectivity of bisphenol formation in the acid catalyzed condensation of hydroxyaromatic compounds with aldehydes or ketones.
In one aspect the present invention relates to A method for making a bisphenol, said method comprising contacting a mixture comprising a hydroxyaromatic compound and a ketone or an aldehyde with an acidic catalyst at a temperature in a range between about 25xc2x0 C. and about 95xc2x0 C. in the presence of a silylmethanethiol promoter having structure I: 
wherein
R1 and R2 are each independently hydrogen, a C1-C40 aliphatic radical, a C3-C40 aromatic radical, a C3-C40 cycloaliphatic radical, or
R1 and R2 together form a C3-C40 cycloaliphatic radical or a C4-C40 aromatic radical;
R3-R5 are each independently a C1-C40 aliphatic radical, a C3-C40 aromatic radical, or a C3-C40 cycloaliphatic radical; or
any two of the groups R3-R5 together form a C5-C40 cycloaliphatic radical or C5-C40 aromatic radical; or
the groups R3-R5 together form a C9-C40 cycloaliphatic radical or C10-C40 aromatic radical.
The present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention and the examples included herein. In this specification and in the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings.
The term xe2x80x9cthiol promoterxe2x80x9d as used herein refers to a molecule incorporating a thiol (SH) group. The thiol promoter acts to improve either one of, or both, the rate and selectivity of bisphenol formation when a hydroxyaromatic compound is condensed with an aldehyde or ketone in the presence of an acidic catalyst relative to the same reaction carried out in the absence of the thiol promoter.
The term xe2x80x9csilylmercaptanxe2x80x9d as used herein refers to a molecule incorporating a thiol (SH) group and a silicon atom. xe2x80x9cBPAxe2x80x9d is herein defined as bisphenol A and is also known as 2,2-bis(4-hydroxyphenyl)propane, 4,4xe2x80x2-isopropylidenediphenol and p,p-BPA.
xe2x80x9co,p-BPAxe2x80x9d is herein defined as o,p-bisphenol A and is also known as 2-(2-hydroxyphenyl)-2-(4-hydroxyphenyl)propane and 2,4xe2x80x2-isopropylidenediphenol.
As used herein the term xe2x80x9caromatic radicalxe2x80x9d refers to a radical having a valency of at least one and comprising at least one aromatic group. Examples of aromatic radicals include phenyl, pyridyl, furanyl, thienyl, imidazolyl, naphthyl, phenylene and biphenyl groups. The term includes groups containing both aromatic and aliphatic components, for example a benzyl group. Further, a C3-C40 aromatic radical is an aromatic radical comprising between 3 and 40 carbon atoms. The 2-imidazolyl group (i) 
illustrates a C3 aromatic radical.
As used herein the term xe2x80x9caliphatic radicalxe2x80x9d refers to a radical having a valency of at least one and comprising a linear or branched array of atoms which is not cyclic. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of aliphatic radicals include methyl, methylene, ethyl, ethylene, hexyl, and hexamethylene groups.
As used herein the term xe2x80x9ccycloaliphatic radicalxe2x80x9d refers to a radical having a valency of at least one and comprising an array of atoms which is cyclic but which is not aromatic. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen. Examples of cycloaliphatic radicals include cyclcopropyl, cyclopentyl cyclohexyl and tetrahydrofuranyl groups.
As used herein the term xe2x80x9ccarbamyl groupxe2x80x9d refers to a functional group comprising the array of atoms OCONH. For example a carbamyl group is present the product of reaction of an alcohol with an isocyante as illustrated by the compound 1-naphthyl methylcarbamate, CAS No. 63-25-2.
As used herein the term xe2x80x9cBoc groupxe2x80x9d refers to a an amine protecting group comprising the tertiary-butoxycarbonyl moiety. The combination of a nitrogen atom bearing both a hydrogen atom and the Boc group is an example of a carbamyl group.
The instant invention provides a method of preparing a bisphenol, such as bisphenol A, by acid catalyzed condensation of a hydroxyaromatic compound, such as phenol, with an aldehyde, such as butanal, or ketone, such as acetone, in the presence of a silylmethanethiol promoter having structure I.
In one embodiment of the present invention at least one of the groups R1-R5 contains a basic functional group by which the silylmethanethiol promoter may be attached to a solid catalyst such as a sulfonated polystyrene catalyst by virtue of a strong hydrogen bonding interaction between the silylmethanethiol promoter containing said basic functional group and the catalyst. The silylmethanethiol promoter is thus immobilized on the solid catalyst thereby avoiding the necessity of introduction of the silylmethanethiol promoter with the feed stream, and the eventual recovery of the silylmethanethiol promoter from the product stream. Functional groups which are sufficiently basic to facilitate ionic attachment of the promoter to a polymeric acid catalyst include, but are not limited to amino and pyridyl groups. In some instances the thiol promoter does not contain a functional group which is basic per se, yet the thiol promoter does contain a functional group capable of forming one or more hydrogen bonds with sulfonic acid (SO3H) or sulfonate (SO3xe2x88x92) groups present in the polymeric acid catalyst, said hydrogen bonds being sufficiently strong to immobilize the thiol promoter on the polymeric acid catalyst. Examples of said functional groups present in the thiol promoter being amido, imido and carbamyl groups as are found in amides, imides and carbamates respectively.
Silylmethanethiols having structure I are known in the chemical 
literature and methods for their preparation have been described in, for example, J. Org. Chem. 53(5) 844 (1987); J. Org. Chem. 51(18) 3428 (1986); and Tetrahedron Letters 26 (11) 1425 (1985). In some instances silylmethanethiols having structure I are commercially available as in the case of trimethylsilylmethanethiol which is available from TCI Chemical Company, Portland, Oreg. Other members of this class may be prepared by reaction of a chloromethylsilane having structure II with a sulfur nucleophile such as sodium sulfide, sodium thioacetate or thiourea. Where sodium sulfide is employed the silylmethanthiol I is obtained directly. Where the thioacetate is employed as a nucleophile, acetate derivative III is obtained. The acetate derivative III is readily converted to the corresponding thiol I upon solvolysis, for example upon heating acetate derivative III with methanol in the presence of a basic catalyst such as triethylamine.
Chloromethylsilicon compounds corresponding to structure II may be prepared by a variety of methods, among them the hydrosilylation reaction of olefins having structure IV with a chloromethylsilanes V incorporating a silicone hydride function. Thus, olefin IV in which radical R6 corresponds to a two carbon lower homolog of radical R5 may be reacted with chloromethylsilane V in the presence of a noble metal catalyst to afford chloromethylsilicon derivative II.
A variety of reaction conditions may be employed for the conversion 
of chloromethylsilanes II into silylmethanethiols I and silylmethanethiol acetate derivatives III. Typically, the chloromethylsilane is combined in a polar solvent such as methanol or dimethylformamide with a slight excess of sodium sulfide or sodium thioacetate and the mixture us stirred at a temperature between about 0xc2x0 C. and about 100xc2x0 C. until all of the starting chloromethylsilane has been consumed as judged by gas chromatography, thin layer chromatography or like analytical technique. Thereupon, the reaction mixture may be distributed between water and a solvent such as toluene or ethyl acetate. The organic layer is then washed with water to complete the removal of inorganic salts and then dried over a suitable drying agent such as magnesium sulfate. Filtration and solvent evaporation affords the crude product which may be employed as a promoter for BPA production in its crude state, or purified, for example by column chromatography or recrystallization, prior to such use.
Examples of silylmethanethiols which may be used as promoters for bisphenol production include, but are not limited to, trimethylsilylmethanethiol, triethylsilylmethanethiol, tripropylsilylmethanethiol,tributylsilylmethanethiol, 1-trimethylsilyl-1-ethylmethanethiol and 1-trimethylsilyl-1-benzylmethanethiol. Where the silylmethanethiol is to be used as a bulk promoter, that is a promoter which is not adapted for attachment to a solid acid catalyst via a strong hydrogen bonding interaction or other covalent bond, the preferred silylmethanethiol promoter is trimethylsilylmethanethiol owing to its availability, ease of preparation and recovery or removal from the reaction product.
In some instances it may be advantageous to employ a silylmethanethiol derivative such as trimethylsilylmethanethiol acetate in the process of the present invention. Under such circumstances it is believed that the silylmethanethiol acetate is converted to the active silylmethanethiol promoter under the reaction conditions. Silylmethanethiol acetates and other silylmethanethiol derivatives which afford a silylmethanethiol under the reaction conditions are advantageously employed at about the same levels as the silylmethanethiol itself.
In some instances the silylmethanethiol promoter may function as an attached promoter, as is the case of those silylmethanethiols which incorporate an amine function which may be used to form an attachment to a solid phase catalyst such as a sulfonated polystyrene. The attachment of the silylmethanethiol to the solid phase catalyst may be based upon a strong hydrogen bonding interaction or a covalent bond. Examples of silylmethanethiols which may be used as attached promoters for bisphenol production include: 3-aminopropyldimethylsilylmethanethiol, 3-N-methylaminopropyldimethylsilylmethanethiol, 3-N,N-dimethylaminopropyldimethylsilylmethanethiol, 3-(1-piperadinyl)propyldimethylsilylmethanethiol, and 2-(4-pyridyl)ethyldimethylsilylmethanethiol.
In addition to silylmethanethiols which incorporate a free amino function, derivatives of such materials incorporating acylated thiol groups and protected amine groups may also be employed. Thus, the N-Boc-3-aminopropyldimethylsilylmethanethiol acetate derivative VI is found to function as an effective attached promoter for bisphenol production. Thiol acetate VI is believed to be converted to 3-aminopropyldimethyl-silylmethanethiol VII under the conditions 
used to prepare bisphenols.
As noted, the instant invention provides a method of preparing a 
bisphenol by acid catalyzed condensation of a hydroxyaromatic compound with an aldehyde or ketone in the presence of a silylmethanethiol promoter having structure I. Examples of hydroxyaromatic compounds include phenol, o-cresol, m-cresol, 2-t-butylphenol, 2-propylphenol and 1-naphthol. Examples of aldehydes include formaldehyde, acetaldehyde, propionaldehyde and butanal. Examples of ketones include acetone, cyclohexanone; 3,3,5-trimethylcyclohexanone and 2-butanone.
A wide variety of bisphenols; such as 2,2-bis(4-hydroxyphenyl)propane; 2,2-bis(4-hydroxy-3-methylphenyl)propane; 1,1-bis(4-hydroxyphenyl)cyclohexane; 1,1-bis(4-hydroxy-3-methylphenyl)cyclohexane and 1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane, may be prepared by the method of the present invention. The present invention is best exemplified by its use in the preparation of bisphenol A via condensation reaction of phenol with acetone. A mixture comprising phenol and acetone is contacted with an acidic catalyst and silylmethanethiol promoter at a temperature between about 20xc2x0 C. and about 100xc2x0 C., preferably between about 40xc2x0 C. and about 90xc2x0 C. and still more preferably between about 50xc2x0 C. and about 80xc2x0 C.
Typically, the acidic catalyst will be a either a sulfonated polystyrene derivative comprising structural units VIII or a non-polymeric mineral acid or an organic sulfonic acid or a fluorinated carboxylic acid. Polymeric acidic resins comprising structure IV are exemplified by Amberlyst(copyright) 131, Amberlyst(copyright) 15 and Amberlyst(copyright) 36, all of which are strongly acidic ion exchange resins available from the Rohm and Haas Company. 
Other suitable polymeric acidic catalysts which may be used in the preparation of bisphenol A include Nafion(copyright) perfluorinated acidic resins available from the Dupont Company. In some instances it may desirable to employ a non-polymeric acid as the catalyst. Examples of suitable non-polymeric acid catalysts include HCl, HBr, HI, BF3, HF, MeSO3H and CF3CO2H.
Where the catalyst employed is a polymer supported acidic catalyst the reactants, phenol, acetone and silylmethanethiol promoter may be passed through a catalyst bed in a continuous fashion. Alternatively, a catalyst may be pretreated with the silylmethanethiol promoter comprising a functional group capable of a strong hydrogen bonding interaction with the acidic catalyst. In the case of a silylmethanethiol attached to a solid acidic catalyst by a strong hydrogen bonding interaction, the preferred loading of said silylmethanethiol on the solid acid catalyst is in a range corresponding to between about 10 and about 60 percent of the acid sites present in the solid catalyst. For a sulfonated polystyrene originally comprising about 5 milliequivalents of SO3H groups per gram of resin, between about 10 and 60 percent of the acid sites corresponds to a loading of silylmethanethiol of between about 0.5 and about 3.0 milliequivalents per gram of resin.
Typically the feed is introduced at a weight hourly space velocity of from about 0.1 to about 6, preferably from about 0.3 to about 3, and even more preferably from about 0.2 to about 1.6 pounds of the feed mixture per pound catalyst per hour. The feed mixture comprises from about 0.1 to about 10 weight percent acetone and about 70 to about 99 weight percent phenol, preferably about 3 to about 8 weight percent acetone and about 85 to about 96 weight percent phenol, and still more preferably about 3 to about 6 weight percent acetone and about 90 to about 96 weight percent phenol. The amount of silylmethanethiol promoter is preferably in a range between about 5 and about 100 millimoles per liter (mmol/L) of feed, preferably about 10 to about 75 mmol/L and even more preferably about 20 to about 40 mmol/L of feed.
The reaction may be also conducted using a soluble acidic catalyst such as methanesulfonic acid. The reaction may be carried out as a batch or continuous process. The reactants are charged to a stirred reactor adapted as desired for batch or continuous operation. Typically a feed solution containing phenol, acetone and promoter are introduced and the acidic catalyst is added separately. The amount of acidic catalyst employed is such that the reaction mixture contains from about 10 to about 1000 mmol of acidic catalyst per liter, preferably from about 20 to about 500 mmol of acidic catalyst per liter, and still more preferably from about 100 to about 300 mmol of acidic catalyst per liter. The reaction mixture comprises from about 0.1 to about 10 weight percent acetone and about 70 to about 99 weight percent phenol, preferably about 3 to about 8 weight percent acetone and about 85 to about 96 weight percent phenol, and still more preferably about 3 to about 6 weight percent acetone and about 90 to about 96 weight percent phenol. The amount of silylmethanethiol promoter is preferably in a range between about 5 and about 100 millimoles per liter (mmol/L) of reaction mixture, preferably about 10 to about 75 mmol/L and even more preferably about 20 to about 40 mmol/L.